LIFE HISTORY OF A TROUT PARASITE. 
349 
all of whom were more or less interested in the so-called “ wormy trout” of the lake, 
and were especially anxions to know something of the cause of the malady, I found 
that with an occasional exception it was necessary for me to j^reface any remarks I 
had to make in answer to queries by giving a short disquisition on tapeworms in 
general before I could make it clear that I was not a raving theorist when I stated 
that the cause of the wormy trout was probably the wormy pelican. 
Since this report is likely to fall into the hands of some who may.be interested 
enough to read it, but whose previous reading on the subject of tapeworms has never 
extended be.vond the obtrusive headlines of some quack advertisement, I have deemed 
it expedient to give a very brief account of the life history of a typical and familiar cestod 
worm, following this with an equally brief statement of what has been found out with 
respect to the life history of a near relative of Dibothriuni eordiceps, before giving an 
account of what is the probable round of life of the latter worm. 
The cestods, then, are a peculiar natural order of worms, all the members of which 
are parasitic during all or at least the greater part of their existence. As a rnle, two 
animals of different kinds, and related to each other as eater and eaten, are required 
to enable the cestod to complete its life history. One of these is called the final and 
the other the intermediate host, the cestod being in each case a more or less unwelcome 
guest. 
Probably the best-known member of this order is the common pork tapeworm 
{Twnia solium), whose intermediate host is usually the pig, in whose flesh it passes the 
larval or encysted stage of its life, constituting the so-called “ bladder worm” or 
“measles” of measly pork. In order to attain the adult stage it is absolutely neces- 
sary that the bladder worm be swallowed by the proper animal. In this case the 
proper animal is man, in whose intestine the bladder worm becomes the adult tape- 
worm. The pig is therefore an intermediate and man a final host of the common tape- 
worm. The life history of the tapeworm proceeds, therefore, in this wise: Man, par- 
taking of improperly cooked, pork containing larval tapeworms, swallows one of the 
latter, which, being liberated by the action of the digestive fluids from the cyst of con- 
nective tissue in which it is inclosed, soon finds a lodgment on the walls of the duode- 
num or other part of the small intestines, where it clings by means of an exceedingly 
small head provided with minute hooks and four small sucking disks. The body gives 
rise to a chain of joints or segments, which rapidly mature, and are voided with their 
contained eggs in the natural way, and, under certain conditions not necessary to detail, 
find their way into the stomach of the pig. A minute embryo is there developed from 
each egg, which penetrates the walls of the stomach or intestine, burrows through 
the tissues, and finally comes to rest, usually in the muscular tissue, where it becomes 
encysted and develops into the bladder worm. Thus the humble round of its passive, 
though somewhat eventful, life is complete. 
The worm which infests the trout belongs to the genus Dihotlirium, or as it is fre- 
quently written, Bothriocephalus. It has been known for some years that certain forms 
related to this genus, which as larvm, known by the name Ligulce, infest many of the 
European fresh-water fishes, more especially the Cyprmidee, reach their final or mature 
stage in a variety of aquatic birds. This has been demonstrated by the experiments 
of Duchamp and Donnadieu, who succeeded in raising mature Dihothria in the intes- 
tines of ducks, which had . been fed Ligulce from the Tench. The migrations in this 
case, as made out by Donnadieu, are as follows ; The eggs develop in the water. 
